Organosilsesquioxanes are silicon-oxygen based frameworks having the general formula (RSiO1.5)n, in which n is an even number≧4. Organosilsesquioxanes which have a very specific structure, for example a compound having the formula (RSiO1.5)8 has an octahedral cage structure, are referred to in the field as organooligosilsequioxanes or polyhedral oligomeric silsesquiloxanes (or POSS®).
Organosilsesquioxanes have the potential to offer good mechanical properties, for example as coatings with good abrasion resistance, and can be formulated to have good chemical resistance, for instance as embodied by hydrolytic stability or stability to UV degradation. These and other properties render the organosilsesquioxanes useful as protective coatings for a wide variety of substrates, particularly polymer-based materials, such as acrylic polymers and polycarbonates, which are routinely used as alternatives to glass in many situations where the weight, tendency to shatter or expense of glass contraindicates its use.
A number of processes for the manufacture and modification of organosilsesquioxanes is described in the literature, including:
(1) the manufacture of partially or fully condensed structures;
(2) the functionalisation of partially or fully condensed structures; and
(3) the manufacture of structures containing more than one organic functionality.
Examples of such processes are disclosed by Brown et al, J. Am. Chem. Soc. (1965) 87:4313-4317 and Feher en al, J. Am. Chem. Soc. (1989)111:1741-1748. In general terms, these processes the use of very low monomer concentrations in solvent and the addition thereto of large amounts of water, to allow the very slow production, typically over a number of months, of specific and defined cage structures. As a result, the materials produced have limited application, for instance in the field of protective coatings, due to the considerable expense associated with their slow production.
The incorporation of functional organic ligands into the basic organosilsesquioxanes structure has been achieved through a number of routes. In this context, the term “functional” is used to describe organic groups that act to impart particular mechanical and/or chemical properties to the final material. For example, the incorporation of organic ligands has been achieved by:
(1) exchange of functionality in an existing fully condensed structure;
(2) cleavage and insertion of functionality into a fully condensed structure;
(3) addition of functionality into a partially condensed structure; and
(4) incorporation of functionality during the fabrication of the structure.
Examples of these processes are described in U.S. Pat. No. 5,047,492 and U.S. Pat. No. 5,484,867.
The last of the above routes, (4), is rarely performed, but where it is it involves co-hydrolysis and condensation of silane precursors containing non-functional organic groups and silane precursors containing functional organic groups particularly with the ability to polymerise. However, the preparation of these functionalised organosilsesquioxanes remains slow.
It would, therefore, be desirable to provide a process for the preparation of organosilsesquioxanes, and compositions comprising the same, which is not subject to the lengthy reaction times described in the prior art, and which therefore makes these materials available to the industry on a more cost-effective basis.